Estimations of environmental flow requirements in a tropical region

Tropical regions are characterised by a high variance in precipitation and experience significant fluctuations in river discharge. These fluctuations in discharge play a dominant role in shaping environmental flow (EF), which is vital for the survival of river ecosystem. However, EF assessment has never been made mandatory for water infrastructure construction and water withdrawal activities in some of the developing countries, including Malaysia. This, in the future, may lead to the severe disturbance of the river ecosystem due to the positive human population growth that requires more needs for water.

　Estimation from global EF can be referred to; however, it was found to have a poor correlation with local estimates; thus, the assessment needs to be conducted locally. Besides, the determination of EF for ungauged basins is still considered a difficult problem. Most frequently, hydrological modelling is used for this purpose prior to any further calculation. Thus, through this study, we aim to determine if there are consistent relationships in EF requirements across different basin sizes that will assist in scalable management strategies.

　The hydrology-based method introduced by Smakhtin and Anputhas  (IWMI, 2006) was selected to calculate EF requirements at 62 sites in Malaysia with available observed records for more than 20 years. The results showed that, on average, 43% of the annual discharge was needed by the river to sustain its ecology, with the assumption that the basins are maintained in fair conditions. In the most regulated conditions, at least 20% of the annual discharge needs to be reserved to maintain the function of rivers as water bodies where, in this case, the ecosystem has significantly modified. We also found that the basin area well corresponds to the discharge and EF with a determination coefficient higher than 0.95. Therefore, it can be acknowledged that the suggested equation (0.0146 × basin area in km² + 2.90) may be used for determining EF in any river reach in the tropical regions, especially Malaysia.

　The estimated EF corresponding to basin management serves as a preliminary basis for sustaining river ecosystem. Overall, our study provides a reference for water practitioners and policymakers, especially for the rapid judgement on the quantity of water resources that need to be conserved.